Apparatus for obtaining silicon from fluosilicic acid

ABSTRACT

Apparatus for producing low cost, high purity solar grade Si wherein a reduction reaction, preferably the reduction of SiF 4 , by an alkali metal (Na preferred) is carried out by jetting a spray of reactants into a reaction chamber at a rate and temperature which causes the reaction to take place far enough away from the entry region to avoid plugging of reactants at the entry region and wherein separation in the melt is carried out continuously from the reaction and the Si can be cast directly from the melt. The melt separation is provided by openings in the reaction chamber wall between about 2 to about 3.5 millimeters in width. The Si is retained within the reaction chamber due to its surface tension.

ORIGIN OF INVENTION

The U.S. Government has rights in this invention pursuant to JPL/DOEContract No. 954471-NAS 7-100 awarded by the U.S. Department of Energy.This invention together with the inventions described in the aboverelated applications evolved (in-part) from research efforts aimed atpreparing low cost, high purity silicon for solar cells. The results ofthat research are contained in the following reports prepared forJPL/DOE:

Quarterly Progress Report No. 1, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. J. Kapur and L. Nanis, August1976;

Quarterly Progress Report No. 2 and 3, "Novel DuplexVapor-Electrochemical Method for Silicon Solar Cell", by: V. J. Kapurand L. Nanis, March 1976;

Quarterly Progress Report No. 4, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. J. Kapur L. Nanis, and A.Sanjurjo, January 1977;

Quarterly Progress Report No. 5, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell"; by: V. J. Kapur, L. Nanis, and A.Sanjurjo, February 1977;

Quarterly Progress Report No. 6, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. J. Kapur, L. Nanis, and A.Sanjurjo, March 1977;

Quarterly Progress Report No. 7, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. J. Kapur, L. Nanis, and A.Sanjurjo, April 1977;

Quarterly Progress Report No. 8, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. j. Kapur, L. Nanis, and A.Sanjurjo, February 1978;

Quarterly Progress Report No. 9, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. J. Kapur, L. Nanis, A. Sanjurjo,and R. Bartlett, April 1978;

Quarterly Progress Report No. 10, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. J. Kapur, L. Nanis, K. M.Sancier, and A. Sanjurjo, July 1978;

Quarterly Progress Report No. 11, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. Kapur, K. M. Sancier, A.Sanjurjo, S. Leach, S. Westphal, R. Bartlett, and L. Nanis, October1978;

Quarterly Progress Report No. 12, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: L. Nanis, A. Sanjurjo, and S.Westphal, January 1979;

Quarterly Progress Report No. 13, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: L. Nanis, A. Sanjurjo, K. Sancier,R. Bartlett, and S. Westphal, April 1979;

Quarterly Progress Report No. 14, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: L. Nanis, A. Sanjurjo, and K.Sancier, July 1979;

Quarterly Progress Report No. 15, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: L. Nanis, A. Sanjurjo, and K.Sancier, November 1979;

Draft Final Report, "Novel Duplex Vapor-Electrochemical Method forSilicon Solar Cell", by: L. Nanis, A. Sanjurjo, K. Sancier, and R.Bartlett, March 1980; and

Final Report, "Novel Duplex Vapor-Electrochemical Method for SiliconSolar Cell", by: L. Nanis, A. Sanjurjo, K. Sancier, and R. Bartlett,March 1980.

The subject matter of the aforementioned reports are incorporated hereinby reference.

REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 569,439 filed on Jan. 9, 1984now U.S. Pat. No. 4,597,948.

Other copending U.S. patent applications relating to the general subjectmatter of this invention, assigned to the same assignee and incorporatedherein by reference are as follows:

Process and Apparatus for Obtaining Silicon from Fluosilicic Acid, Ser.No. 337,136 filed Jan. 5, 1982 by Angel Sanjurjo;

Process and Apparatus for Casting Multiple Silicon Wafer Articles, Ser.No. P-1559 filed even date herewith by Leonard Nanis;

Process and Apparatus for Obtaining Silicon from Fluosilicic Acid, Ser.No. P-1563 filed even date herewith by Kenneth M. Sancier;

Process and Apparatus for Obtaining Silicon from Fluosilicic Acid, Ser.No. P-1564 filed even date herewith by Kenneth M. Sancier;

Process and Apparatus for Obtaining Silicon from Fluosilicic Acid, Ser.No. P-1572 filed even date herewith by Leonard Nanis and Angel Sanjurjo;and

Process and Apparatus for Obtaining Silicon from Fluosilicic Acid, Ser.No. P-1700 filed even date herewith by Angel Sanjurjo.

BACKGROUND OF THE INVENTION Field of Invention

Silicon is, at present, the most important material in modernsemiconductor technology and is finding increased use in solar cells forthe photovoltaic generation of electricity. In view of the importance ofthe solar cell application, the stringent requirements for purity andlow cost and further in view of the orientation of the work done, theprocess and apparatus is described primarily in the context ofproduction of silicon for solar cell use. However, it is to beunderstood that both the process and apparatus used are generally usefulin the production of silicon for whatever end use, as well as othertransition metals such as Ti, Zr, Hf, V, Nb and Ta.

A major deterrent to the development of practical solar photovoltaicsystems is the cost of high purity silicon. With todays technology,approximately twenty percent of the total cost of a silicon solar cellis ascribed to the silicon material alone. That is, the cost of thesilicon material produced by the conventional hydrogen reduction ofchlorosilanes constitutes at least twenty percent of the cost ofproducing the cell. It is estimated that the cost of the silicon must bereduced by almost an order of magnitude before silicon solarphotovoltaic panels will prove to be economically feasible as a powersource. The fact that the chlorosilane processes require multipleseparations, which are so energy intensive and require such largecapital investments, indicate that cost of the silicon can not bereduced sufficiently to make silicon solar cells economically feasiblewithout a major process change. As a consequence, an approach to theproduction of solar grade silicon which is less complex, less energyintensive and which requires less capital equipment is required.

Technical Field of the Invention

It has been found that silicon of more than sufficient purity to meetthe solar cell applications can be produced within the economicrequirements from the metallic reduction of silicon fluoride.Preferably, the silicon fluoride is prepared from an aqueous solution offluosilicic acid, a low cost waste by-product of the phosphatefertilizer industry by treatment with a metal fluoride whichprecipitates the corresponding fluosilicate. This salt is filtered,washed, dried and thermally decomposed to produce the correspondingsilicon tetrafluoride and metal fluoride which can be recycled to theprecipitation step. The silicon tetrafluoride is then reduced by asuitable reducing metal and the products of reactions are treated toextract the silicon. Each of the steps is described in detail usingsodium as typical reducing agent, and sodium fluoride as typicalprecipitating fluoride but the concept applies as well to other reducingmetals and metal fluorides that can reduce silicon fluoride and formfluosilicates. The process in one form is described in detail in anarticle entitled Silicon by Sodium Reduction of Silicon Tetrafluorideauthored by A. Sanjurjo, L. Nanis, K. Sancier, R. Bartlett and V. J.Kapur in the Journal of the Electrochemical Society Vol. 128, No. 1,January 1981 and the subject matter of that article is specificallyincorporated herein by reference.

Background

There are available systems for the production of silicon utilizing someof the reactions of the present system. For example, Joseph Eringer inU.S. Pat. No. 2,172,969 describes a process wherein sodiumsilico-fluoride is mixed with sodium in powder form and placed in acrucible which is heated and in the upper part of which two pieces ofcopper wire gauze are placed parallel to each other. The space betweenthe pieces of gauze, which can also be heated, is filled with copperwool. When the crucible has been filled and closed, it is heated toabout 500° C. At this temperature, reaction takes place and silicon andsodium fluoride are formed whereby the silicon which is mechanicallyexpelled by the sudden increase in pressure is collected in chambers ortowers connected to the furnace. The equation of the reaction is asfollows:

    Na.sub.2 SiF.sub.6 +4Na=Si+6NaF

or this can be expressed:

    Na.sub.2 SiF.sub.6 =SiF.sub.4 +2NaF

    SiF.sub.4 +4Na=Si+4NaF

After the reaction product has been cooled at least to 200° C. it isfinely divided and is treated with water or heat treated with dilute 1:1sulfuric acid. Hydrogen fluoride gas is liberated (which latter can thenbe made into hydrofluoric acid or a metallic fluoride) metallicsulphates are produced and the silicon separates out on the surface inamorphous form as shining metallic froth.

The reaction expressed in equation form is:

    Si+6NaF+3H.sub.2 SO.sub.4 =Si+6HF+3Na.sub.2 SO.sub.4

After the silicon has been separated from the metallic sulphatesolution, it is again washed and is dried at 80° C. The silicon obtainedin this way is in the form of a impalpable redish or grey-brown powderwhich discolors strongly and which, even if the raw products wereimpure, contains a minimum of 96-97% silicon. The yield amounts to about87% of the theoretically possible yield.

Robert Aries reports in U.S. Pat. No. 3,041,145 that attempts made toreduce silicon halides by the use of sodium vapor have not led to acommercially successful process. He gives as an example the processdiscussed in the Eringer patent, supra, and points out 96%-97% purity isentirely outside the range of purity required for silicon to be used forphotocells, semiconductor rectifiers, diodes, and various types ofelectronic equipment. As has already been discussed, the conventionalhydrogen reduction of chlorosilanes especially with the electrolyticdeposition techniques used, is too energy intensive to be economical.

Aries ascribes the purity problem to impurities in the sodium used inthe reduction reaction and teaches that further elaborate and expensivepurification of the purest available commercial grade sodium is requiredto produce silicon of solar or semiconductor grade. More recently, V. J.Kapur in U.S. Pat. No. 4,298,587 also supports the view that suchpurification is required. In fact, this patent teaches that both thesodium and the silicon tetrafluoride must be purified using an energyintensive technique comparable to the electrolytic deposition systems ofthe chlorosilane reduction processes.

It has been determined that silicon of the desired grade is obtainedwithout the elaborate purifications of commercial grade sodium orsilicon tetrafluoride obtained from the fluosilicic acid (from thereaction shown above) provided the reduction reaction is carried out insuch a way that it goes to completion, the proper environment ismaintained during the reduction reaction and the product is properlyisolated from contaminating atmosphere and container walls until thereaction is complete and solid silicon which is below reactiontemperature is formed and separated. In copending patent applicationentitled Process and Apparatus for Obtaining Silicon from FluosilicicAcid, Ser. No. 337,136 filed Jan. 5, 1982 by Angel Sanjurjo and assignedto the present assignee, the isolation from the container is carried outusing a powdered substance so that the reaction product does not adhereand can be removed by a simple dumping process. The system is successfulbut generally is not needed in connection with the melt separation ofthe present process.

The present invention is directed to the part of the process which dealswith the manner of carrying out the reaction between SiF₄ and analkaline earth metal (eg. Na) to produce Si. In carrying out thereaction, finely divided reactants are jetted into the reaction chamber.Both U.S. Pat. No. 4,188,368 to Wolf et al and U.S. Pat. No. 4,102,765to Fey et al deal with reactions where Si is produced usingly finelydivided injected feed stocks. Keeton U.S. Pat. No. 4,169,129 disclosesan apparatus and process for pulse feeding a fine spray of liquid Nainto a Si production reactor. Bagley U.S. Pat. No. 2,995,440 and BakerU.S. Pat. No. 3,069,255 disclose procedures for introducing molten Nainto a reaction vessel with chlorides of titanium, while Hill U.S. Pat.No. 2,890,953 discloses a procedure for adding atomized liquid Na in areactor (also with chlorides of titanium). Maurer (U.S. Pat. No.2,941,867) separately charges a reducing metal reactant and a halide ofa high melting metallic element from group II, III, IV, V and VI of theperiodic table both in the fluid state into an externally cooledreaction zone. However, none of these patents deal successfully with theimportant problem of feed nozzle plugging.

In order to appreciate the problem, consider (as noted above) that whenliquid Na at 150° C. contacts SiF₄, a rapid exothermic reaction takesplace. The Na burns in the SiF₄ atmosphere to produce Si and NaF. SinceNa melts at 98° C., in principle, liquid Na at temperatures below 140°C. can safely be drop-fed into a reactor kept under a constant SiF₄pressure. The reaction takes place at the bottom of the reactor which iskept at temperatures above 200° C. Experimentally, it is observed thatdue to the heat generated by the reaction, the Na injection nozzleoverheats and the reaction takes place at the nozzle causing a build upof reaction products which plug the nozzle, and thus, the Na feedingsystem. Further, the reaction products produced by the systems disclosedin the patents are in a form that is difficult to separate. In view ofthe stringent purity requirements for solar grade silicon, separationtechniques that tend to introduce impurities are distinctlydisadvantageous.

The present invention is specifically concerned with performing theNa/SiF₄ reaction in such a manner that it takes place far enough awayfrom the entry region of the reactants that they are freely introduced.In a preferred embodiment, the reaction is performed in such a mannerthat the reaction products (Si and NaF) are easily separated by meltseparation and the Si continuously cast.

Summary and Objects of Invention

In carrying out the present invention sodium fluosilicate Na₂ SiF₆ isprecipitated from fluosilicic acid followed by thermal decomposition ofthe fluosilicate to silicon tetrafluoride SiF₄. The SiF₄ is then reducedby an alkali metal, preferably Na, to obtain silicon which is separatedfrom the mix, preferably by melt separation. The reduction reaction iscarried out by jetting finely divided reactants into a reaction chamberat a rate and temperature which causes the reaction to take place farenough away from the injection or entry region so that there is noplugging at the entry area and thus, the reactants are freelyintroduced. Preferably the reaction is carried out in such a manner thatthe resulting reaction products (Si and NaF) are easily removed andseparated directly and continuously from the melt and the Si may bedirectly cast.

The invention has for its principal object the provision of a processfor obtaining silicon of sufficient purity to produce solar photovoltaiccells inexpensively enough to make their use practical.

A further object of this invention is to provide a process by means ofwhich silicon can be obtained which is substantially free of impuritiesstarting with relatively inexpensive and impure fluosilicic acid.

A still further object of this invention is to provide a process forproducing Si wherein SiF₄ and a reductant, preferably Na, are introducedinto a reactor in finely divided form and at a rate and temperature thatcauses the reduction to take place at a location removed from the entryarea so that the reaction products do not prevent introduction of eitherof the reactants into the reactor.

Another object of the invention is to provide a process for producingsolar grade Si by reaction of SiF₄ and a reductant in such a manner thatSi is separated from the reaction products continuously and directly.

Yet another object of the invention is to provide a reactor vessel forseparating Si from the molten reaction products of a Si producingreaction.

Still a further object of the invention is to provide a process and anapparatus for continuously separating Si in molten form from the moltenreaction products and casting the Si into a continuous sheet as it isseparated.

The novel features which are believed to be characteristic of theinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation, together with further objects and advantages thereof may bestbe understood by reference to the following description taken inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating a preferred embodiment of theprocess for producing high purity silicon by the melt process;

FIG. 2 is a graph illustrating the time, temperature and pressurecharacteristics of the silicon fluoride and sodium reaction showing timein minutes plotted along the axis of abscissae and temperature indegrees C. and pressure (torr) plotted along the axis of ordinates;

FIG. 3 is a somewhat diagrammatic central vertical section through areactor unit and the Si continuous casting arrangement showing reactionproduct separation means and details of one embodiment of the Na andSiF₄ feed mechanism according to the present invention;

FIG. 4 is a diagrammatic central vertical section through anotherreaction unit similar to the one of FIG. 3 showing another form ofreaction product separating reactor and Na/SiF₄ feed mechanism accordingto the present invention; and

FIG. 5 is a perspective view showing details of the reaction chamber ofthe embodiment of FIG. 4 and particularly showing how the slits (152)which drain the NaF and separate it from the Si are arranged to form anelectrical resistance heater from the reaction chamber wall (151).

DESCRIPTION OF PREFERRED EMBODIMENTS

A preferred embodiment of the process for production of pure siliconstarting with inexpensive commercial grade fluosilicic acid isillustrated in the flow diagram of FIG. 1. The overall process consistsof three major operations which encompass a series of steps. The firstmajor operation (shown in brackets 10 in the drawing) includes the stepof precipitation of sodium fluosilicate from fluosilicic acid followedby generation of silicon tetrafluoride gas. The second major operation(brackets 12 on the drawing) comprises the reduction of silicontetrafluoride to silicon (30), preferably by sodium, and the thirdoperation (brackets 14) involves the separation of silicon from themixture of silicon and sodium fluoride.

Consider first the steps for generation of silicon tetrafluoride(operation 10). The preferred starting source of silicon is an aqueoussolution of fluosilicic acid (H₂ SiF₆), a waste product of the phosphatefertilizer industry, that is inexpensive and available in largequantities. Fluosilicic acid of commercial grade [23 weight percent(w%)] has also been used directly as received without purification orspecial treatment and is shown as the silicon source 16 in FIG. 1. Asanother alternative, fluosilicic acid is obtained by treating silica, orsilicates (natural or artificially made) with hydrogen fluoride. TheSiF₆ ⁻² is then precipitated in sodium fluosilicate Na₂ SiF₆, by addinga sodium salt to the solution (step 18). Other salts such as NaF, NaOH,NaCl, or similar salts of the elements in groups IA and IIa of theperiodic table are all candidates. The major selection criteria are, lowsolubility of the corresponding fluosilicate, high solubility ofimpurities in the supernatant solution, high solubility of theprecipitating fluoride salt, and non-hygroscopic character of thefluosilicate.

Based on these criteria, the preferred fluosilicates in order ofpreference are Na₂ SiF₆, K₂ SiF₆ and BaSiF₆. Using the preferred NaF asthe precipitating salt, the hydrogen of the fluosilicic acid isdisplaced by the sodium to form sodium fluosilicate, a highly stable,nonhygroscopic, white powder, and sodium fluoride which is recycled. Inequation form the reaction is

    H.sub.2 SiF.sub.6 +2NaF=Na.sub.2 SiF.sub.6 +2HF

As an example, Sodium fluosilicate was precipitated by adding solidsodium fluoride directly to the as received commercial grade fluosilicicacid 18. The yield was a supernatant liquid containing mostly HF andsome NaF and H₂ SiF₆ along with the sodium fluosilicate. HF is alsogiven off (20). The supernatant fluid was removed and the sodiumfluosilicate washed with cold distilled water to remove any remaining HFand H₂ SiF₆. After filtering and drying in an oven at 200 degrees C., aminimum yield of 92% of pure sodium fluosilicate 22 (determined by x-raydiffraction) was obtained. The product sodium fluosilicate is anonhygroscopic white powder that is very stable at room temperature andthus provides an excellent means for storing the silicon source beforeit is decomposed to silicon tetrafluoride.

Precipitation under the just described conditions acts as a purificationstep, with most impurities in the original fluosilicic acid staying insolution. This effect is increased by adding suitable complexing agentsto the fluosilicic acid solution previous to the precipitation. Bothinorganic complexing agents such as ammonia and organic agents such asEDTA (ethylenediaminetetraacetic acid) help to keep transition metalions in solution during precipitation of the fluosilicate.

The fluosilicate is thermally decomposed 24, thus,

    Na.sub.2 SiF.sub.6 =SiF.sub.4 +2NaF

to give the solid sodium fluoride, which is recycled 26, and to generatethe SiF₄ gas 28. The decomposition does not take place appreciably attemperatures below 400° C. Therefore, impurities which are volatile atthis temperature can easily be removed by a vacuum treatment below thistemperature. The decomposition of Na takes place at temperatures between500° and 700° C. Impurities left in the solid phase are typicallytransition metal fluorides such as Fe, Ni, Cu, etc., whose volatility attemperatures below 700° C. is very low and therefore do not contaminatethe SiF₄ gas. The gas thus produced can be fed directly to the reductionreactor or it can be stored for future use.

In separate experiments, it was determined that SiF₄ gas at a pressureof 0.4 atm is in equilibrium at 650° C. with solid Na₂ SiF₆ and NaF.Therefore, as SiF₄ is needed, the Na₂ SiF₆ is thermally decomposed(FIG. 1) at 650° C. in a graphite-lined, gas-tight stainless steelretort. Gaseous SiF₄ evolved at 650° C. was condensed as a white solidin a storage cylinder (cooled by liquid nitrogen) attached to theretort. The SiF₄ gas was allowed to expand by warming of the storagecylinder to room temperature and was fed into the reactor as needed.SiF₄ gas prepared in this manner was determined by mass spectrometricanalysis to be more pure than commercial grade SiF₄, as shown in TableI. Ions formed from the sample gas were identified from the observedmass numbers, isotopic distribution and threshold appearance potentials.The detection limit was better than 0.005%. Positively identifiedgaseous impurities are listed in Table I; no metallic impurities weredetected. Peaks corresponding to B compounds, such as BF₃, werespecially checked, but none were found.

                  TABLE I                                                         ______________________________________                                        Mass spectrometric analysis of SiF.sub.4                                                SiF.sub.4 prepared                                                  Ion       from H.sub.2 SiF.sub.6 (%)                                                                 SiF.sub.4 commercial (%)                               ______________________________________                                        SiF.sub.3.sup.+                                                                         96.9         93.6                                                   Si.sub.2 OF.sub.6.sup.+                                                                 3.04         4.24                                                   SiOF.sub.2.sup.+                                                                        (--)         1.79                                                   CCl.sub.3.sup.+                                                                         (--)         0.159                                                  SiO.sub.2 F.sub.2.sup.+                                                                  0.076       0.098                                                  Si.sub.2 O.sub.2 F.sub.4.sup.+                                                          (--)         0.081                                                  SiO.sub.2.sup.+                                                                         (--)         0.035                                                  ______________________________________                                    

Although the SiF₄ produced from H₂ SiF₆ has less impurity, thecommercial grade SiF₄ was also used for experimental convenience. Thepossible presence of metallic impurities in commercial SiF₄ wasdetermined by bubbling the gas through high purity water and treatingthe resulting slurry with an excess of HF to drive off Si as SiF₄. Thefinal clear solution was then analyzed by plasma emission spectroscopy(PES). The results are listed in Table II, together with PES analysis ofthe waste by product H₂ SiF₆ and the NaF used to precipitate Na₂ SiF₆(18 and 22 FIG. 1). Comparison of the first two columns of Table II withcolumn three shows that the concentration of some elements, e.g., Li,B.V. Mn, Co, K, and Cu, were unchanged by precipitation of Na₂ SiF₆whereas the elements Mg, Ca, Al, P, As, and Mo were diminished by afactor of 5-10. Some elements were concentrated into the Na₂ SiF₆,namely Cr, Fe, and Ni. The fourth column in Table II is representativeof the impurity content to be found in SiF₄ gas prepared on a commercialscale. The low content of P is of special significance for bothsemiconductor and solar cell applications. Elements known to reducesolar cell efficiency (V, Cr, Fe, Mo) are uniformly low in commercialgrade SiF₄. Only Mn, As, and Al are of comparable concentration in bothNa₂ SiF₆ and SiF₄ at the 1 parts per million (ppm) by weight or lesslevel.

                  TABLE II                                                        ______________________________________                                        Plasma emission spectroscopy analysis, ppm (wt)                               Element   H.sub.2 SiF.sub.6                                                                     NaF        Na.sub.2 SiF.sub.6                                                                   SiF.sub.4                                 ______________________________________                                        Li        0.1     (--)       0.2    0.01                                      Na        460     (--)       (--)   1.8                                       K         9.0     (--)       8.0    0.3                                       Mg        55      (--)       6.4    2.3                                       Ca        110     10         18     1.6                                       B         1.0     (--)       0.8    <0.01                                     Al        8.0     <2.5       1.3    1.2                                       P         33      (--)       5      0.08                                      As        8.8     (--)       0.2    0.28                                      V         0.3     <5         0.3    <0.01                                     Cr        0.8     <3.5       8.8    <0.01                                     Mn        0.2     <4         0.4    0.16                                      Fe        13      <7         38     0.04                                      Co        0.54    (--)       0.7    <0.01                                     Ni        1.17    <8         4.2    <0.01                                     Cu        0.12    <4         0.6    <0.01                                     Zn        1.4     (--)       1      <0.01                                     Pb        14.5    (--)       5      0.03                                      Mo        11      (--)       1.0    <0.01                                     ______________________________________                                    

SiF₄ /Na reaction, the central operation of the pure Si process,(FIG. 1) is the reduction of SiF₄ by Na according to the reaction

    SiF.sub.4 (g)+4Na(l)=Si(s)+4NaF(s)

This reaction is thermodynamically favored at room temperature, however,it has been found experimentally that Na has to be heated to about 150°C. before any appreciable reaction can be observed. Once the reactionhas been initiated the released heat raises the temperature of thereactant (Na) which in turn increases the reaction rate. Under adiabaticconditions, a temperature of 2200 K. is predicted for the reaction withthe stoichiometric quantities of SiF₄ and Na. In practical reactors,rapid consumption of gaseous SiF₄ produces a pressure decrease. Thekinetic behavior of the Na-SiF₄ reaction is complex because of theinterplay of several factors, e.g., pressure of SiF₄, vaporization ofNa, local temperature, porosity of two solid products, and transport ofSiF₄ and Na vapor through the product crust that forms on the liquid Na.

Although only preliminary studies have been made of the kinetics, thegeneral features of this reaction have been surveyed. In a series ofexperiments to estimate reaction temperature 5 grams of Na were loadedin a Ni crucible (3 cm ID, 4 ccm high) and heated in SiF₄ initially at 1atm pressure. The Na surface tarnished at around 130° C., with theformation of a thin brown film. As the temperature increased, the colorof the surface film gradually changed from light brown to brown andfinally to almost black. The SiF₄ /Na reaction became rapid at160°+/-10° C. and liberated a large amount of heat, as indicated by asudden rise in reaction temperature. The pressure in the reactortypically decreased slightly until the temperature increased sharply,with an associated rapid decrease in SiF₄ pressure. The reaction lastsfor several seconds only (until the Na is consumed). For SiF₄ pressuresbelow 0.3 atm the reaction mass was observed to glow at a dull red heat.For higher pressure, a characteristic flame was observed. The shortestreaction time (20 sec) and the highest temperatures (about 1400° C.)were obtained when the initial pressure of SiF₄ was around 1 atm. Inaddition, complete consumption of Na was obtained for 1 atm SiF₄. Whenscale-up of this reaction was attempted by loading larger amounts of Na,it was found that as the depth of the Na pool increased, the amount ofNa remaining unreacted also increased. The product formed a crust on topof the Na surface, building a diffusion barrier for the reactants. Asthe barrier thickness increased, the reaction slowed and eventuallystopped.

For separation (operation 14 FIG. 1) of the silicon from the products ofreduction, in the preferred melt separation process embodiment of thisinvention, the products are heated until a melt is formed and the NaF isdrained off (36) leaving the Si (34) which can if necessary be furtherpurified. The melting and separation process is described in detailbelow in connection with the scaled up system. Leach separation isdescribed in the copending application Ser. No. 337,136, previouslyreferenced. In the leach process, the silicon and sodium are removed andcombined with water and a selected acid. The resultant silicon and watersoluble sodium fluoride are then separated.

On the basis of studies of the parameters that affect the reaction, asystem was developed that is shown with various reactant feedarrangements and reaction product separation configurations in thecentral vertical sections through the reactors of FIGS. 3 and 4. It isnoted that all of the reactant feed arrangements are specificallydesigned to cause the reduction reaction to take place far enough awayfrom the feed system (into the reactor) positively to prevent build upof reaction products at the entry and, thus, avoid any possibility ofplugging of the entering reactants.

The upper section 40 of the reactor system, shown somewhat schematicallyin FIG. 3, constitutes a reactant (Na and SiF₄) dispenser and the lowersection 42 is the reactor section where the reaction takes place. Inthis illustrated embodiment, the reactant dispenser section 40 includesa stainless steel liquid sodium injection tube 44 vertically andcentrally located in the top flange 46 of the reactor. The Na injectiontube 44 is provided with a conventional stainless steel belows valve 48for controlling Na flow into the reactor section 42. The inner diameterof the Na injection tube 44 is selected to provide the desired Najetting action and to assure an appropriate Na stream ejection velocityand Na stream size.

In order to bring the reactants together in the reaction zone and toprovide some cooling of the entering liquid Na, a SiF₄ feed head 50 ispositioned concentrically around the entry portion of the Na ejectiontube 44 with its ejection aperture 52 positioned to feed SiF₄ into thereactor concentrically around the Na stream 54. The SiF₄ is fed into thereactor at room temperature and its entry is controlled by a constantpressure valve 56 in order to keep pressure constant in the reactor atfrom about 0.5 to about 5 atmospheres. That is, as SiF₄ is fedconcentrically with the Na into the hot zone of the reactor, the SiF₄-Na reaction takes place depleting SiF₄. The depletion in turn activatesthe constant pressure valve 56, thus, feeding more SiF₄ into thereactor. The resultant gas flow keeps a relatively constant temperatureat the injection region by its cooling action and by producing a jetthat keeps hot particles of reaction products from reaching the nozzleend of Na feed tube 44. Keeping the reaction products away from thereactant entry area in this manner eliminates plugging of the injectionapertures.

This mode of operation increases the rate of Si production. Using a Nanozzle exit aperture of 0.005 inch and jetting Na above its reactingtemperature of 150° C., the reaction took place far enough away from theentry area to prevent plugging for injection temperatures up to 250° C.No plugging occurs for reaction temperatures up to 900° C.

The reduction reaction (FIG. 1 operation 12) takes place in the lowerreactor section 42 of the reactor system. As previously noted, thereduction reaction is highly exothermic and therefore, temperaturecontrol is desirable to help prevent reaction products from moving upnear the reactant injection area. Such control prevents reactants fromplugging the injection nozzle and preventing reactant injection. Withtemperature control in mind, the top reactant injection nozzlesupporting flange 46 for the reactor 42 is cupped (58) or recessed(upwardly in the drawing) to thermally isolate the injection area fromthe hotter regions of the reactor 42. Further isolation from the hotreaction region is provided by inserting a disk like toroidal heatinsulating baffle 60 between the reaction zone and the nozzle thus,effectively forming a semi-isolated nozzle entry chamber 62. Thecentrally located aperture 64 in the baffle 60 is of a size to letreactants from the nozzle enter the reaction zone, prevent the injectedproducts from spraying the outer reactor walls and minimize heattransmission between the reaction area in the reactor 42 and the nozzleentry chamber 62.

Additional temperature control is provided by water cooled tubing 66which extends around the cupped portion 58 of the nozzle supportingflange 46. In this connection, note again that it has been foundexperimentally that Na reacts with SiF₄ only above 150° C. Therefore, aslong as the cooling coils 66 in cooperation with the reactant inputtemperatures, jet velocities and heat shielding baffle 60 maintain thereactant feed area below this temperature, premature reaction at thefeed port and nozzle plugging is prevented. Further to the point ofpreventing premature reaction, the SiF₄ is preferably injected at atemperature of between -86° C. and 120° C. and the liquid Na is jettedat a temperature of between 98° and 130° C.

It is contemplated that the reaction products (NaF and Si) will beseparated by a melt process at temperatures above the melting point ofSi (1412° C.) and preferably in the range between 1450° and 1500° C. Itis also contemplated that the separation and removal of the reactionproducts will take place on a continuous basis. The physical structureand configuration of the reactor portion 42 of the system is designed toproduce such results.

As illustrated, the reactor section 42 includes a double containerarrangement with an outer generally cylindrical container 70, designedto capture and dispense the liquid NaF reaction product, surrounding aninner cylindrical reaction product receiving and separating container72. In order to withstand the high temperatures involved and to avoidcontaminating the reaction products, the inner container 72 is composedof high purity graphite and in order to perform the separation ofreaction products, the container walls are made with small continuousperforations. For simplicity, the perforations are not shown in thedrawing. The bottom 74 of the container 72 is generally conical in shapewith a solid non-porous cylindrical molten Si removing drain pipe 76 inthe center and thus, has the appearance of a common funnel. The drainpipe 76 is shown closed by a movable drain plug 78. The conditionillustrated is for a normal run in process with reaction products builtup and some melt separation products in place.

The Na and SiF₄ are jetted into the inner container 72 through theaperture 64 in the heat insulating baffle 60 and reaction starts to takeplace in the hot upper reaction zone 80. As the reaction proceeds, apool 82 of reacted and partially reacted Na and SiF₄ form where thereaction goes to completion (pool described above). Immediately belowthe pool of reaction products 82, a hotter melt separation zone 84 isformed. The melt separation zone 84 is maintained at a much highertemperature (means of heating explained below) than the reactionproducts zone 82 above it and the reaction products effectively meltout. At these temperatures, ie temperatures above 1412 degrees C., thereaction products (Si and NaF) are liquids which are separable becausethe NaF will normally float on top of the Si. That is, the liquid Si,which is more dense than NaF, agglomerates and settles to the bottom ofthe reactor vessel 72. Liquid NaF, which melts at 993° C., is immisciblewith Si and usually wets graphite in the presence of liquid Si. The moreor less spherical globules 88 of Si dispersed in the NaF 88 and thelarge Si ingot or pool 90 at the bottom, as illustrated in FIG. 3, lookmuch like those actually found in a sectioned graphite container afterthe reaction products, heated to the temperatures contemplated here,have been allowed to cool (solidify). It is apparent that, while molten,the NaF both coats the Si and wets the graphite, thus, providing abarrier which prevents the Si from reacting with the graphite and avoidsany impurity transfer or migration through and from reactor walls.

Due to its relatively high surface tension (relative to NaF), Si remainsin a porous or perforated container while the low surface tension NaFflows out the pores or perforations provided the pores are of the propersize for the temperatures of the reaction products. It has beendetermined experimentally that for the melt zone temperaturescontemplated, perforations in the walls of the inner reaction productreceiving and separating container 72 of between 2 and 3.5 millimeters(mm), the NaF flows through the perforations while the molten Si remainsin the contaner 72. The average dimension of the perforations 79 may befrom less than 0.5 mm to about 3 mm or greater, preferably between about0.2 mm to about 3.5 mm, more preferably between about 1 mm to about 3.5mm, and most preferably between about 2 mm to about 3.5 mm. If theperforations are appreciably smaller than 2 mm, the NaF does notdischarge well unless pressure is applied and for apertures appreciablygreater than 3.5 mm Si has a tendency to enter and interfere with NaFdischarge. The Si is removed by extracting the movable closure plug 78to allow the Si to flow out of the reaction container drain pipe 76. Theflow is adjusted so that the process is continuous. That is, the flow ofSi out the pipe 76 is adjusted so that the reduction reaction iscontinuously taking place in the reaction zone 80 and reaction productscontinuously settle through the reaction product zone 82 and into themelt separation zone 84 with NaF continuously flowing out the perforatedinner reaction container 72 and Si agglomerating at the bottom and beingcontinuously withdrawn from the drain pipe 76.

The generally cylindrical outer container 70 of the reactor section 42performs the functions of collecting and dispensing the NaF (separatedreaction product), physically supporting the inner graphite reactionproduct receiving and separating container 72 and supporting bothinduction heating elements which heat the inner container 72 and theinsulation which minimizes radiation heat loss. The functions performedby the outer container 70 in large measure prescribe the characteristicsof the material and its structure. For example, the fact that thecontainer 70 collects and dispenses the NaF reaction product which seepsthrough the perforations in the inner reaction chamber 72 makes isdesirable to make the the container of a material which will not sloughoff, react with the hot NaF, or in any way introduce contaminates whichwould prevent the NaF from being recycled without being purified. It isalso desirable that the outer container 70 be spaced far enough from theinner container 72 to provide free flow for the NaF. The walls of thetwo containers 70 and 72 are held in their spaced relationship at thetop by means of an annular graphite ring 96 which snuggly surrounds theinner container 72 near its top and fits tightly inside the outercontainer 70 and at the bottom by means of the reaction container drainpipe 76 which is sealed in the exit aperture 98 in the bottom 100 of theouter container 70. The NaF is discharged through a drain pipe 104 atthe bottom (right side in drawing) of the outer container 70.

The outer container 70 must also have sufficient physical strength toperform its support functions and be nonconductive to allow the internalreaction chamber 72 to be inductively heated by heating coils 92 whichsurround the lower melt separation zone 84. Silicon carbide (SiC) is amaterial which meets the criteria for the container 70. However, it mustbe made in such a way that it has a high enough resistivity to allow theinduction heating coils 92 to heat the internal reaction chamber. If SiCis used, it should be lined with graphite, Si or anothernoncontaminating powder as taught in the copending Sanjurjo patentapplication Ser. No. 337,136, supra., to assure that no contaminants areadded to the reaction products from the container walls. Beryllia (BeO)is also a high strength ceramic which meets all of the above criteriafor the outer container 70.

For heating efficiency and to reduce heat radiation from the reactor,the heating coils 92 are covered with a silica felt insulating 94 (1.3cm thick). The top flange 46 of the reactor 42, the flange whichsupports the reactant injection nozzles (Na injection tube 44 and SiF₄feed head 50), mates and is sealed to an outwardly extending lip orflange 102 around the top of the outer reaction chamber or vessel 70.

The Si discharged via the exit pipe 76 may be treated in any number ofways. For example, it may be cast into ingots (see copending patentapplication P-1559 Ser. No. 435,718) but, as illustrated, it iscontinuously cast into sheet by a preferred technique. In order to formthe Si cast sheet 106, the Si exit pipe 76 (below the blocking plug 78)discharges the molten Si into a graphite silicon forming and castingsection 108 where the central aperture necks down in one dimension toform a Si discharge channel 110 that is rectangular in cross section.Since the Si in the inner reaction product receiving and separatingcontainer 72 is in the presence of NaF salt and the salt wets the Si butnot the graphite (as explained above) formation of SiC is prevented. Asthe Si is discharged through the rectangular channel 110 in the Siforming and casting section 108 the salt (NaF) coats and isolates(coating designated 112) it from the container wall throughout itslength.

In order to assure solidification of the Si 106, a longitudinaltemperature gradient (hot to cold from top to bottom) is imposed alongthe length of the casting section 108. For this purpose, electricalheating coils 114 are provided around the casting section 108 and inorder to conserve heat, the silica felt insulation 94 also extends downaround both the coils 114 and casting section 108. The temperaturegradient is such that the Si 106 gradually cools and solidifies (freezesto a solid sheet) as it travels down the channel but the salt (NaF)coating 114 remains molten. In the embodiment illustrated, thetemperature gradually decreases along the length of the casting section108 from above the melting point of Si (1412° C.) to just above themelting point of the salt (993° C. for NaF).

As the NaF coated Si passes out of the exit pipe 76 into the rectangularSi discharge and casting channel 110 the temperature is allowed to dropto about 1412° C. (the melting point of Si) and gradually decreasesalong the length of the channel to about 1200° C. (well above themelting point of NaF and well below the melting point of Si). As thesalt coated Si travels down the channel 110 it freezes into a solidsheet 106 surrounded by molten salt 112. The Si 106 expands uponsolidification and is cushioned by the surrounding molten salt layer112. The cushion thus provided prevents breaking either the solid sheetSi or the graphite forming and casting section 108. The length of therectangular Si discharge channel 110 and the temperature gradient alongit can be optimized for the desired crystalline structure of the sheetSi.

The salt coating 112 on the discharged sheet Si 106 is easily removed bya conventional aqueous leaching process. For example, the salt coating112 is readily removed in 1.0N acid solution.

For a discussion of aqueous leaching of Si, see copending Sanjurjopatent application Ser. No. 337,136 entitled Process and Apparatus forObtaining Silicon from Fluosilicic Acid filed Jan. 5, 1982 and assignedto the assignee of the present invention. The sheet casting system hasthe added advantage that the molten coating salt acts as a purifyingagent. This effect adds to the versatility of the casting technique.That is, the fact that the additional purification is obtained allowsthe casting technique to be used with relatively inexpensive feed gradesof Si that would not normally be considered for a solar grade endproduct. Where the casting process is used apart from the meltseparation process which, as illustrated, already uses the NaF salt,other salts which wet Si but not the container walls may be used. It hasbeen found that either Na₂ SiO₃ or NaF or a combination of the twoperform both the wetting and the purification functions.

Another feed head or nozzle 116 for introducing the reactants into areaction chamber in a way that avoids plugging by preventing reactiontoo near the entry region is illustrated in FIG. 4. As in the previousfigure, the upper section 40 of the reactor system, shown somewhatschematically in FIG. 4, constitutes a reactant (Na and SiF₄) dispenserand the lower section 42 is the section where the reaction takes place.The "shower head" nozzle design shown allows excellent use of thereactor volume.

The nozzle design in this embodiment is called a "shower head" designbecause of the similarity of the central Na feed portion 118 of itsnozzle to a conventional bathroom shower head. That is, the Na feedportion 118 includes a conventional stainless steel belows valve 122controlled Na inlet piper 120 that feeds directly into an enlargedoutlet head portion 124 closed at its lower end with a Na feed disc 126having many small Na outlet apertures 128. Additional entry Natemperature control is provided by surrounding the upper part of the Nafeed tube 120 and enlarged outlet head portion 124 with a sealed oil orargon confining chamber 130 which is provided with a valved (entry valve132) entry tube 134 and valved (exit valve 136) oil or argon exit tube138. The argon temperature controlling gas is recycled through atemperature regulating cooler (not shown).

The shower head Na feed nozzle 116 is centrally located, supported andsealed in a disk-like top 140 for the reactor section 42. Asillustrated, the SiF₄ for the reaction is brought in and directedinwardly into the jetted streams of Na from the apertures 128 of the Nafeed nozzle 116. In order to accomplish this, SiF₄ is brought in by wayof stainless steel tubing 142 through a conventional constant pressurevalve 144 into a double walled cone shaped ejection nozzle 146 whichsurrounds the shower head Na injection nozzle 116. The double walledcone-shaped ejection nozzle 146 looks much like the cone shaped walls oftwo slightly spaced but nested funnels. In this way, the SiF₄ isinjected at room temperature and uniformly from all angles around thejetting streams of Na.

Entry of SiF₄ is controlled by the constant pressure valve 144 in orderto keep pressure constant in the reactor at from about 0.5 to about 5atmospheres. That is, as SiF₄ is fed concentrically with the Na into thehot zone of the reactor, the SiF₄ -Na reaction takes place depletingSiF₄. The depletion in turn activates the constant pressure valve 144,thus, feeding more SiF₄ into the reactor. The resultant gas flow keeps arelatively constant temperature at the injection region by its coolingaction and by producing a jet that keeps hot particles of reactionproducts from reaching the nozzle end (the apertures 128 of Na outletdisk 126). Keeping the reaction products away from the reactant entryarea in this manner eliminates plugging of the injection apertures 128.

This mode of operation increases the rate of Si production and allowscontinuous Si production. In the embodiment illustrated, the inlet Nafeed tube 120 is 3/8 inch SS, the enlarged section or head portion 124is 1 inch in diameter and the Na feed apertures 128 in the Na feed disk126 are between 2 and 5 one-hundredths of an inch in diameter. In thisembodiment the liquid Na feed is maintained at a pressure between 1.5and 2 atmospheres and a temperature between 98° C. and 130° C. so thatit jets out the Na feed apertures 128 at a velocity sufficient toprevent any plugging reaction product build-up at the nozzle 116. Usingthese parameters and jetting Na below its reacting temperature of 150°C., the reaction will take place far enough away from the entry area toprevent plugging for injection temperatures up to 250° C. No pluggingoccurs for reaction temperatures up to 900° C.

The reduction reaction (FIG. 1 operation 12) takes place in the lowerreactor section 42 of the reactor system. Again, as in the embodiment ofFIG. 3, it is contemplated that the reaction products (NaF and Si) willbe separated by a melt process at temperatures above the melting pointof Si (1412° C.) and preferably in the range between 1450° and 1500° C.It is also contemplated that the separation and removal of the reactionproducts will take place on a continuous basis. The physical structureand configuration of the reactor portion 42 of the system is designed toproduce such results.

As illustrated, the reactor section 42 includes a double containerarrangement with an outer generally cylindrical container 148, designedto capture and dispense the liquid NaF reaction product, surrounding aninner cylindrical reaction product receiving, melting and separatingcontainer 150. In order to withstand the high temperatures involved andto avoid contaminating the reaction products, the inner container 150 iscomposed of high purity graphite and in order to perform the separationof reaction products, the cylindrical container wall 151 is made withnarrow slits 152. As illustrated in the perspective view of FIG. 5, thenarrow slits 152 are arranged so that the container wall also forms acontinuous serpentine conductor which acts as a resistive heater forpurposes of raising the temperature of the reaction products. For theliquid NaF freely to flow out of the slits 152 in the wall 151 while theSi globules remain in the container 150, the slits 152 are made between2 and 3.5 millimeters in width and for the purpose of forming thiscontinuous serpentine conductor, the slits 152 are made alternately fromthe top and bottom of the wall 151. A separation of the container wall151 is made by one cut 154 that extends completely from top to bottom ofthe wall and is filled with an insulator 156 which can be a ceramic suchas berylia or a high temperature graphite felt. In this way a voltage(source not shown) can be applied between ends of the serpentine wallconductor to cause inner reaction product receiving container 150 toheat the reaction products (resistively) to the desired temperature.

The bottom 158 of the container 150 has the general appearance of acommon funnel in that it is conical in shape with a solid non-porouscylindrical molten Si removing drain pipe 160 in the center. The drainpipe 160 is shown closed by a movable drain plug (not shown). Note thatin order to prevent the funnel like bottom 158 of the container 150 fromshorting across the slits 152 of the container wall, the bottom 158 iseither made of a non conducting material or, as illustrated, isinsulated by a ring of insulating material 162, such as graphite felt,between the bottom 158 and the wall 151. The condition illustrated isfor a normal run in process with reaction products built up and somemelt separation products in place.

The Na and SiF₄ are jetted into the inner reaction product receivingcontainer 150 and reaction starts to take place in the hot upperreaction zone 80. Note that the reaction takes place in exactly themanner as described with respect to the embodiment of FIG. 3 and themelt separation proceeds in the same way. Therefore, the reactionproducts are given identical reference numerals in both Figures and arenot again described in detail here. As the reaction proceeds, a pool 82of reacted and partially reacted Na and SiF₄ form where the reactiongoes to completion (pool described above). Immediately below the pool ofreaction products 82, a hotter melt separation zone 84 is formed. Themelt separation zone 84 is maintained at a much higher temperature(means of heating explained above) than the reaction products zone 82above it and the reaction products effectively melt out.

At these temperatures, i.e., temperatures above 1412 degrees C, thereaction products (Si and NaF) are liquids which are separable becausethe NaF will normally float on top of the Si. That is, the liquid Si,which is more dense than NaF, agglomerates and settles to the bottom ofthe reactor vessel 150. Liquid NaF, which melts at 993° C., isimmiscible with Si and usually wets graphite in the presence of liquidSi. More or less spherical globules 86 of Si dispersed in NaF 88 and alarge Si ingot or pool 90 at the bottom, as illustrated, are formed.

It is apparent that, while molten, the NaF both coats the Si and wetsthe graphite, thus, providing a barrier which prevents the Si fromreacting with the graphite and avoids any impurity transfer or migrationthrough and from reactor walls. Due to its relatively high surfacetension (relative to NaF), Si remains in the container 150 while the lowsurface tension NaF flows out the slits 152 provided the slits 152 areof the proper size. Hydraulic pressure dictates the proper slit size andthe volume and temperature of the reaction products determines thehydraulic pressure. It has been determined experimentally that for themelt zone temperatures contemplated, and slits 152 in the wall 151 ofthe inner reaction product receiving and separating container 150 ofbetween 2 and 3.5 millimeters (mm), the NaF flows out while the moltenSi remains in the inner container 150. If the slits 152 are appreciablysmaller than 2 mm, the NaF does not discharge well and for slitsappreciably greater than 3.5 mm Si has a tendency to enter and interferewith NaF discharge.

The Si is removed by means of the reaction container drain pipe 160. Theflow is adjusted so that the process is continuous. That is, the flow ofSi out the pipe 160 is adjusted so that the reduction reaction iscontinuously taking place in the reaction zone 80 and reaction productscontinuously settle through the reaction product zone 82 and into themelt separation zone 84 with NaF continuously flowing through the slits152 in the wall 151 of inner reaction container 150 and Si agglomeratingat the bottom and being continuously withdrawn from the drain pipe 160.Although it is feasible to use a number of different kinds ofreceptacles to receive the Si, the sheet casting arrangement shown anddescribed in connection with FIG. 3 is a preferred embodiment. Both forsimplicity and brevity the apparatus is not illustrated or describedagain here.

Like the outer container 70 of FIG. 3, the generally cylindrical outercontainer 148 of the reactor section 42 performs the functions ofcollecting and dispensing the NaF (separated reaction product),physically supporting the inner graphite reaction product receiving andseparating container 150 and supporting insulation 164 which minimizesradiation heat loss. Unlike the outer container 70 of FIG. 3, outercontainer 148 may be conductive since the internal reaction chamber 152is a heater. Graphite is an excellent choice of material.

Again, the outer container 148 must be spaced far enough from the innercontainer 150 to provide free flow for the NaF. The walls of the twocontainers 148 and 150 are held in their spaced relationship at the topby means of an annular ring 166 which snuggly surrounds the innercontainer 150 near its top and fits tightly inside the outer container148 and at the bottom by means of the reaction container drain pipe 160which is sealed in the exit aperture 168 in the bottom 170 of the outercontainer 148. The NaF is discharged through a drain pipe 172 at thebottom of the outer container 148.

For heating efficiency and to reduce heat radiation from the reactor,the outer container 148 is covered with a silica felt insulation 174(not shown here). The top flange 140 of the reactor 42, which supportsthe shower head reactant injection nozzle 116 mates and is sealed to anoutwardly extending lip or flange 176 around the top of the outerreaction chamber or vessel 148.

The process sequence shown in FIG. 1 was selected because of theinherent simplicity of the steps and their independent and combinedsuitability for scale-up. Some purification occurs during precipitation(operation 1, FIG. 1) for Mg, Ca, Al, P, and As due to the highsolubility of their fluosilicates and fluosalts. Some concentrationtakes place for Cr, Fe, and Ni, and this effect may be due tocoprecipitation of these elements as fluorides since their fluosilicatesare very soluble. From Table II, it is clear that most of thepurification is accomplished as a result of the thermal decomposition instep 24 (FIG. 1). Most transition metal fluorides are in very stablecondensed phases at the decomposition temperature (650° C.) in step 24(FIG. 1) and, therefore, will stay in the solid. In addition, volatilefluorides formed during the decomposition of fluosalts such as Na₂ TiF₆and Na₂ ZrF₆ will condense upon cooling of the SiF₄ gas stream from step24. The condensed material is then removed from the gas mainstream byin-line fume particle filtration. The presence of any metallic or dopantimpurities was not detected using mass spectrometry (Table I) in eitherthe gas produced in the above reaction or in the commercial SiF₄ gas.The analysis done on the SiF₄ by passing the gas through high puritywater was based on the hypothesis that impurities should be hydrolyzedand/or trapped in the SiO₂ formed.

The results listed in Table II show that the level of metal impuritiesin the resulting SiO₂ is so low that, for practical purposes, the SiF₄can be considered free of metallic impurities. The Na feed, reactormaterials, and possible contamination of the product during handlingremain as possible sources of impurities in the Si.

The impurities in Na can be divided roughly into three types accordingto their tendency to react with SiF₄, as classified by the free energyof reaction. The first type of impurity includes aluminum and elementsfrom the groups Ia, IIa and IIIB. The free energy of reaction of SiF₄with these impurities ranges from -100 to -200 kcal/mole SiF₄ at roomtemperature and from -50 to -100 kcal/mole SiF₄ at 1500K. It isexpected, therefore, that even when these impurities are present at theppm level, they will react with the SiF₄ to form correspondingfluorides. Subsequently, the fluorides will be dissolved preferentiallyin the NaF phase.

The second type impurity includes transition metals such as Mo, W, Fe,Co, Ni, and Cu, and the elements P, As, and Sb. These elements exhibitpositive free energies of reaction in excess of 100 kcal/mole SiF₄ andare not expected to react with SiF₄. However, it is an experimental factthat the silicon resulting from the SiF₄ -Na reaction contains amountsof Fe, Ni, and Cr in proportion to the concentration of these elementsin the Na feed. The mechanism by which these metals are transferred tothe silicon has not yet been studied. In any case, the concentration ofFe, Cr, Ni, and also Ti can be decreased by a factor of about 10⁴ to 10⁶for single-pass directional solidification or the Czochralskicrystal-pulling procedures used presently for solar cell manufacture. Atthe resulting levels, these elements would not be detrimental to solarcell performance.

Boron represents a third type of impurity. The free energy of reactionof this element with SiF₄ is positive but small (5-20 kcal/mole SiF₄ fortemperatures up to 1500K); therefore, some partial reaction can beexpected and B will be distributed between the NaF and Si phases. It isnoted that the levels of the dopant elements B, P, and As in thereaction Si are the same as in the semiconductor grade silicon used asreference or control. Since it is convenient to have dopant levels aslow as possible to permit flexibility in subsequent doping proceduresfor semiconductor and solar cell applications, the low B and P contentof Si produced in this process is of advantage. It is noted that thepurity of the silicon produced by the SiF₄ -Na reaction is, at aminimum, nominally appropriate for solar cell manufacture.

From the foregoing discussion, it will be understood that the objects ofthe invention have been carried in that high purity Si can be preparedand cast using the inexpensive starting materials H₂ SiF₆ and Na.Favorable thermodynamics of the reduction step, easily controlledkinetics, and abundant availability of inexpensive starting materialsmake this method attractive. Of special interest for semiconductorapplications are the low concentrations of B and P impurities in theproduct Si. The Si produced by the SiF₄ -Na reaction, particularly whenpurified further by directional solidification (the casting), should bea low cost material suitable for the manufacture of solar cells andother semiconductor products.

While particular embodiments of the invention have been shown, it will,of course be understood that the invention is not limited thereto sincemany modifications in both process and apparatus employed may be made.It is contemplated that the appended claims will cover any suchmodifications as fall within the true spirit and scope of the invention.

What is claimed is:
 1. A casting member for casting molten siliconflowing therethrough comprising: said casting member defining a centralaperture which necks down in one dimension to form an elongatedrectangular discharge passage therethrough and a cushioning salt coatinglayer 112 disposed between said molten silicon and said casting memberwhereby the silicon flowing through said passage is in sheet form, andmeans to establish a temperature gradient of from above about 1412° C.to above about 993° C. along said casting member whereby said silicontemperature is reduced so as to solidify said silicon as it expands uponsolidification and passes through said casting member in sheet form, andmeans for supplying said central aperture of said casting member withmolten silicon and molten salt a reaction container, said reactioncontainer having a bottom non-porous solid drain pipe in communicationwith said central aperture, said drain pipe having a movable closureplug for controlling the flow of molten silicon and molten salt from thereaction container drain pipe into said central aperture of said castingmember.
 2. A casting member according to claim 1, wherein said castingmember is formed from graphite.
 3. A casting member according to claim2, including means for forming said cushioning salt layer from NaF.
 4. Acasting member comprising means defining a rectangular passage as viewedin horizontal cross section, means for forming a solid sheet of siliconwithin said passage and a means for maintaining a cushioning salt layerbetween the means for forming said silicon and said passage definingmeans and means for maintaining said passage at a temperature of about993° C. or above.
 5. A casting member comprising means defining arectanglar passage as viewed in horizontal section, means for providingsilicon within said passage, means for maintaining a cushioning saltlayer between said silicon and said means for defining a rectangularpassage and means for maintaining temperature along said passage fromabove about 1412° C. to above about 993° C.
 6. A casting membercomprising means defining an elongated rectangular passage as seen inhorizontal cross-section, means for maintaining silicon in a moltenstate within said passage and means for maintaining a molten cushioningsalt layer between said silicon and said means defining said passage,means for maintaining the means defining the passage at a temperatureabove the melting point of said salt layer.
 7. A casting membercomprising means defining a rectangular passage, means for forming asolid sheet of silicon within said passage, and means for maintaining acushioning salt layer between said silicon and said means for definingsaid passage.